Tens of thousands of Orangemen and women took part in 19 marches all over Northern Ireland yesterday to commemorate the Protestant King William of Orange's victory over Catholic King James II at the Battle of the Boyne in 1690.

Tensions were high amid fears that the annual Twelfth of July Orange Order parades could spark serious sectarian violence. Security was tight in Belfast, particularly in Ardoyne where assistant chief constable Alan McQuillan, warned republican paramilitaries were planning a riot along the most controversial route.

Police in riot gear stood in front of 200 Catholic protesters as they hurled verbal abuse, whistled and spat at Orangemen, who marched down the Crumlin Road, past the nationalist area on their outward journey in the morning. But there was no physical contact between the two sides.

Morning marches in two other flashpoint areas, down the Springfield Road in west Belfast and on south Belfast's Ormeau Road, where the parades commission barred Orangemen from crossing the bridge into Lower Ormeau, passed off peacefully.

However, Mr McQuillan took the unusual step on Thursday night of telling the media that republicans were making hundreds of petrol and acid bombs and bussing youths into Ardoyne, to start trouble and attack the police and army. He declined to point the finger directly at the Provisional IRA but implied it by saying it was the main paramilitary group in the area.

Gerry Kelly, of Sinn Fein, hotly denied what he claimed was an "outrageous piece of black propaganda".

He said: "The only strangers are the Orangemen and the hundreds of police and army being bussed in to force an unwanted anti-Catholic parade through a Catholic area."

But the police claimed they were proved right when soldiers seized missiles hidden on the roofs of Ardoyne shops. They include more than 80 spiked objects, 12 20ft steel bars, bottles, paint, petrol and half a tonne of bricks.

Anxieties were also raised by discovery of an incendiary device in a van close to the main parade in Belfast city centre. Security forces were alerted by an anonymous telephone warning and army experts carried two controlled explosions.

Jim Rodgers, a former Belfast Ulster Unionist lord mayor and an Orangeman, said the bombers clearly wanted to kill Orangemen, band members and spectators. Police sources blamed the bomb on dissident republicans but Mr Rodgers was convinced it was the work of the Provisional IRA.

Police linked the group to recent rioting in east Belfast's Short Strand area. Any indications that the provisionals involved in more violence would have damaging ramifications for the peace process, for which unionist support is already at an all-time low.

David Trimble, the Ulster Unionist leader, has given the prime minister until July 24 to come up with confidence-boosting proposals.

But Jeffery Donaldson, the hardline Ulster Unionist Lagan Valley MP, told thousands of Orangemen in Lisburn, Co Antrim, nothing short of the exclusion of Sinn Fein from the Stormont government would restore unionist trust.

"Paramilitaries must no longer be permitted to hold the process to ransom," he said. "We are entitled to ask Tony Blair why Sinn Fein ministers are still in the government of Northern Ireland when there is incontrovertible evidence that the IRA are continuing to use and threaten violence."

Orangemen also heard a resolution condemning all violence, and the order's grand master, Robert Saulters, speaking in Dollingstown, Co Down, castigated Orangemen who attacked police at Drumcree last Sunday.